1. Field of the Invention
The present invention relates to a photosensor. A photosensor is very often used as a photoelectric conversion apparatus for processing image information, for example, in a facsimile and/or a digital copying machine.
2. Related Background Art
As an example of a photosensor, a planar type photoconductive photosensor can be taken which is fabricated by providing a pair of opposing electrodes, the spacing between which forms a photoreceptor, on a photoconductive layer of chalcogenide, CdS, CdSSe or amorphous silicone (hereinafter referred to as the a--Si). Photoconductive photosensors can easily constitute a long line sensor array and is especially recently researched and developed. Among others, a photosensor using an a--Si photoconductive layer is excellent in optical response characteristic and expected as a photosensor which will allow for high speed reading.
FIG. 1 is a plan view of one example of such a photoconductive photosensor. FIG. 2 is a cross-sectional view taken alone the line V--V of FIG. 1. In FIGS. 1 and 2, reference numerals 22, 24 and 26 denote a transparent glass substrate, an a--Si photoconductive layer and an ohmic contact layer, respectively. Reference numerals 28 and 30 denote a pair of electrodes. Like this, there is generally a layer which produces an ohmic contact between photoconductive layer 24 and electrodes 28, 30.
Conventionally, such a photosensor is formed by steps such as those shown in FIG. 3.
Namely, a--Si photoconductive layer 24 is first formed on the upper surface of glass substrate 22 by plasma CVD using as a material a silane gas such as SiH.sub.4 gas. Next, an n.sup.+ layer 26' is formed by plasma CVD on photoconductive layer 24 by using as a material a mixture of the silane gas and a doping gas of PH.sub.3 or AsH.sub.3. A conductive layer 28' of metal such as Al is deposited on n.sup.+ layer 26' by vacuum evaporation or sputtering. (FIG. 3A).
Subsequently, photoconductive layer 28' is patterned by conventional wet etching to form electrodes 28, 30 (FIG. 3B). Exposed n.sup.+ layer 26' portion is removed by etching with electrodes 28, 30 being used as mask to form ohmic contact layer 26 (FIG. 2). (Problems which the Invention is Intended to Solve)
In the above method of producing a conventional photosensor, removal of unnecessary n.sup.+ layer 26' is performed by either wet etching or dry etching such as plasma etching.
When wet etching is used, etch pit is likely occur on the surface of photoconductive layer 24 exposed to an etching solution and the selectivity of etching is considerably large, so that an n.sup.- layer is likely to be formed on the surface of photoconductive layer 24. On the other hand, when dry etching is used, the component of the cathode material will undesiably be implanted. Each of these problems will deteriorate the photosensor characteristic. A common problem to both the etching methods is that these etching methods would roughen the surface of photoconductive layer 24 to reduce the S/N ratio.